The invention concerns a hydraulic dual circuit steering system with
a first circuit having a first control unit and a first steering motor connected with the control unit via working connections,
a second circuit having a second control unit and a second steering motor connected with the control unit via working connections,
a changeover valve, which in a first position activates the first circuit and deactivates the second circuit and in a second position activates the second circuit and deactivates the first circuit.
A hydraulic dual circuit steering system of this kind is known from DE 196 22 731 C2. The changeover valve short-circuits the working connections of the inactive circuit, so that an operation of the belonging control unit will have no effect on the corresponding steering motor. The switching of the changeover valve occurs on each starting and stopping of the vehicle to prevent the valve from getting stuck. During operation a switching always occurs when the pressure in the first circuit drops, thus suggesting that an error has occurred in this circuit.
The not yet published German patent application DE 198 44 331 also describes a hydraulic dual circuit steering system, in which the changeover valve is electrically operable. Both circuits have different control units. In one circuit the control unit is made up of a hydraulic steering unit. In the other circuit a proportional valve is provided. The inlets of both circuits are constantly exposed to pressure, that is, either each of the circuits has its own pump or the circuits are connected in series, so that the tank connection of the first circuit is connected with the pump connection of the second circuit.
Dual circuit steering systems have actually justified their existence. They contribute substantially to the security of vehicles, which is particularly of great importance in connection with heavy working machines, which are permitted for the public traffic.
In practice, however, problems with the switching between the two circuits may occur from time to time. In the circuit switched to, the working or motor connections were short-circuited, that is, for a period they were not under pressure. Due to the leakages, which cannot be avoided in practice, a somewhat lower pressure exists in the section next to the steering motor, which makes itself felt in connection with the switching, for example by means of a jerk in the vehicle. This jerk occurs in that in the steering motor of the just connected circuit the corresponding pressure has to be built up again. A jerk of this kind may in itself effect a wrong reaction from the driver. Even worse is that a driver often interprets such a jerk as an error signal, which in connection with the jerk often causes the driver to perform xe2x80x9cunconscious actionsxe2x80x9d.
The invention is based on the task of improving the steering behaviour.
In a hydraulic dual circuit steering system as mentioned in the introduction, this task is solved in that in the first position of the change-over valve the working connections of the second circuit and in the second position of the change-over valve the working connections of the first circuit are connected with a pressure source.
Thus, the pressure of the individual steering motor is always kept at a level, which is so high that no significant pressure differences appear when switching from one steering motor to the other. The supply to the working connections is in this connection independent of the position of the individual control unit in the corresponding circuit, that is, the pressure is also maintained, when the corresponding control unit is closed. Thus, no additional time will be required to xe2x80x9cfill upxe2x80x9d the working connections through the control unit. On the contrary, the pressure is already available here. Particularly when the working connections are short-circuited on deactivation of the circuit, the pressure exerted on the working connections is not expected to cause a change of the reactions, as the higher pressure is available on both sides of the corresponding steering motor. The steering motors can, but must not necessarily, be mechanically connected with each other.
Preferably, one LS line serves as pressure source. An LS line is a load sensing line, which usually always carries the highest pressure available in the system. Thus it is ensured that the working connections of the inactive circuit are always held at the highest pressure available in the system. It may be assumed that only a small amount of hydraulic fluid is required to maintain the pressure in the working connections of the inactive circuit, as usually only small amounts of fluid are lost through leakages. The capacity of the load sensing line is therefore sufficient without problems.
Preferably, the pressure source is connected with the inactive circuit via a throttle. The throttle ensures that an error in the inactive circuit, for example a large leakage, will not cause a complete loss of the load sensing pressure in the active system.
In a preferred embodiment it is provided that the changeover valve has a valve element with a short-circuiting path for the working connections of the inactive circuit, which is connected with the pressure source. Thus, the changeover valve is not merely used for the short-circuiting of the individual working connections of the inactive circuit, but also to supply the pressure from the pressure source via this short-circuiting path into the connection between the working connections. On the one hand, this saves unnecessary design efforts, on the other hand, however, ensures the individual function in connection with the switching.
It is also advantageous that a control device is provided, which keeps the second control unit in the neutral position, when the changeover valve is in the first position. This is particularly advantageous when the two control units have different embodiments, that is, the second control unit being, for example, a proportional valve. In this case, the control unit of the second circuit is not moved by a steering movement, so that the working connections through this control unit are blocked, that is, closed. Thus, the working connections of the second circuit form a closed system, which can easily be maintained at the desired pressure.
Preferably, both the changeover valve and the second control unit have an electrical drive. This drive can either work immediately electrically, for example as a magnet drive, or it can work with a hydraulic auxiliary force support, for example in the shape of a bridge circuit, whose one diagonal is supplied with a pressure difference, whereas the other diagonal has a drive for the valve element of change-over valve or control unit, respectively. With an electrical drive of this kind, the desired dependence between the changeover valve and the second control unit can relatively easily be established.
In a particularly preferred embodiment an accumulator is provided, which is, at least in the first position of the changeover valve, connected with the pressure source, thus causing an increase of a supply pressure in the second circuit when the changeover valve is switched. Additionally to the xe2x80x9chighxe2x80x9d pressure at the working connections of the second circuit, the accumulator also enables an increase of the supply pressure in the second circuit in the moment of switching. This increase must only be effective for a short while. Thus, it is prevented that after the opening of the second control unit a pressure decrease in the working connections of the second circuit takes place through the control unit.
In this connection it is preferred that the change-over valve has an intermediary position between the first and the second position, in which the accumulator is connected with a control line of a pressure supply device of the second circuit. Thus, the accumulator pressure is not directly switched into the supply line, but the control pressure is increased, for example the pressure in the load sensing line for the pressure supply of the second circuit. This gives a much faster reaction of the supply side, which additionally occurs exactly in the instance of the transition from one circuit to the other. When the second circuit is then rendered active, the pressure in this circuit is at the required high level, both on the xe2x80x9cworking sidexe2x80x9d and on the xe2x80x9csupply sidexe2x80x9d. It may also be provided that the change-over valve remains in the intermediary position for a short period (in the range of milliseconds), so that during this period a connection between the accumulator and the supply device of the second circuit is established. In fact, this period only has to be long enough to enable the accumulator to pass on its pressure accordingly. An alternative to this is that when switching from the first position to the second position, the valve element of the changeover valve permits the corresponding connection for a corresponding period, in spite of the movement.